The rapid and accurate measurement of lead in biologic, environmental and product materials is of major importance in public health and ultimately in the prevention of lead poisoning. Lead poisoning is caused solely by exposure to and absorption of the heavy metal. Lead may exist in various compound forms, such as oxides, halides, carbonates and acetates. Because it is lead itself that is toxic, the chemical and physical form of lead only slightly influences toxicity by modulating absorption and retention.
Lead poisoning is treatable poorly for two reasons: first, it is difficult to remove lead from critical target organs once it is absorbed; and second, some of the target organ damage induced by lead appear to be irreversible.
Prevention of lead poisoning requires two strategies to succeed: early detection of increased absorption and identification of lead in sources. Early detection is based on biological monitoring. Source identification is based on sampling air, drinking water, dusts, soils and food. For many children in the U.S., a major source of lead in dusts and soils is from old paint thereby making the measurement of lead in painted surfaces another priority. Also, for some environmental applications, measuring lead in soils, sediments and wastes is an important requirement.
Screening persons suspected to have been exposed to lead is based on two principles: measuring lead in blood, urine, bone, teeth or hair; and measuring biological markers or early cellular responses to lead. Direct measurement of lead requires collecting appropriate samples, extracting lead from the biological matrix and analyzing using atomic absorption spectrophotometry (AAS). The analytic phase requires laboratory support, investment in and maintenance of fixed laboratory equipment, skilled technical operators and inevitably delay between sampling people and obtaining an analytical result. For example see Pruszkowska et al. (1983) Atomic Spec. 4, pp. 59-61; and Barthel et al. (1973) J. A.O.A.C. V.56, No. 5.
Lead can also be measured in aqueous media, such as water and extracted blood, by electrochemical methods using a lead-specific electrode. The method is known as anodic stripping voltametry and (ASV) relies on the principle of measuring conductance changes associated with the plating and discharge of lead ions in solution. A portable ASV device is available, however, the method is limited in sensitivity.
An alternative method of screening for possible lead exposure was developed in the 1950's and is based on the effects of lead on cellular heme synthesis (in general, see papers in Silbergeld, EK & Fowler, BA (eds.) Mechanisms of Chemical-Induced Porphyrinopathies, Ann. NY Acad. Sci. (1987) v. 514). Lead is known to alter heme synthesis, common to many cells, at several steps. Because the pathway is under considerable internal feedback control, substantial alterations in the amounts of final product (heme) and various intermediates can be measured. Thus, measurement of aminolevulinic acid (ALA), coproporphyrins and protoporphyrin have been utilized as markers for lead poisoning. Also, activity of the rate-limiting enzyme, ALA dehydrase (ALAD), has been measured (see Doss; Marks et al.; Bernard & Lauwerys in Silbergeld and Fowler, supra).
In the early 1970's, Piomelli and coworkers demonstrated that measuring protoporphyrin in red cells was an accessible and reliable marker for lead exposure in children. Since that time, measuring protoporphyrin (frequently called erythrocyte protoporphyrin (EP) or "free" erythrocyte protoporphyrin (FEP)) is the standard method for screening children and adults for lead exposure. The EP assay requires small samples of blood collected by fingerstick and a portable instrument (the hematofluorometer) was built to provide an instant readout for purposes of clinic-based screening. Throughout the 1970's and 1980's, EP screening was the method of choice in public health to detect both childhood and occupational lead exposure.
However, it was known from the 1970's that there were significant limits to the utility of EP as a screening tool. For example, EP is influenced by iron status of individuals and EP does not rise significantly until blood lead levels are above 30 .mu.g/dl. (Micrograms per deciliter is the standard designation for blood lead concentrations; some authorities recommend using the international unit of micromolar, .mu.M; roughly, 20 .mu.g/dl equals 1 .mu.M.) Thus there are limits of both specificity and sensitivity with the EP assay.
Measuring lead in sources requires the appropriate sampling techniques, extraction of lead from environmental media or product matrices and analysis of lead by AAS. Before the development of AAS, lead was measured in a chemical reaction utilizing dithizone and formation of a lead-sulfide complex (see Chisolm et al. (1955) Amer. J. Disease of Children 89, pp. 159-168). Although the method has been replaced almost completely by AAS, a recent home test kit based on the dithizone reaction has been marketed for measuring lead leached from ceramics and cans (Frandon Enterprises). However, the test kit is considered by some to be unreliable.
Lead also can be analyzed by ion-coupled inductive plasma techniques and by x-ray induced fluorescence spectrophotometry. The former method is primarily a research tool and is used in studies, for example, of specific ion monitoring to determine sources of environmental contamination or of human exposure. The latter method has been adapted for use with portable equipment specifically detecting lead in painted surfaces such as interior woodwork. The sensitivity and reliability are less than ideal and furthermore, the equipment requires a radioactive source to generate x-rays.
Clearly current methods of lead assay are far from ideal. The shortcomings become amplified in view of recent developments in knowledge and public policy regarding lead that demand advances in screening and source identification. With respect to screening, it is generally recognized that lead toxicity occurs in children at blood lead levels as low as 10-15 .mu.g/dl. Although occupational health policy has not been updated since 1977 (the publication of the OSHA lead standard), recent data strongly indicates that an appropriate standard for preventing occupational lead poisoning is considerably lower than the current blood lead level of 40 .mu.g/dl (see Silbergeld et al. (1991), New Solutions, in press).
Recent surveys indicate that lead poisoning, defined as exposures resulting in blood lead levels in excess of 10-15 .mu.g/dl in children, is very prevalent in the U.S. (Agency for Toxic Substances and Disease Registry (ATSDR), Report to Congress on the Nature and Extent of Childhood Lead Poisoning, 1988). The problem has causes in the many widespread sources of lead in the environment, including the food supply. As a consequence, the Centers for Disease Control have recommended that all children be screened for lead exposure starting at 6 months of age and continuing at least yearly thereafter until age 6 (U.S. Public Health Service, Strategic Plan for the Elimination of Childhood Lead Poisoning, February 1991). Furthermore, increased surveillance of workers is clearly required to prevent occupational disease (Silbergeld et al., 1991, supra). Soon it is likely there will be recommendations to assess lead exposure in pregnant women and in the elderly based upon new epidemiologic studies on the effects of lead in those populations (Silbergeld, Environmental Health Perspectives 91: 156, 1990).
With respect to source identification, the recognition that lead at low doses is toxic necessarily results in the need to detect levels of lead in environmental and dietary sources at lower levels, and in the need to develop proactive programs of environmental monitoring of media such as dust and soils. As an example, the EPA has lowered the drinking water standard to an advisory level of 5 parts per billion (ppb), one-third of the current standard, and lowered the air standard from 1.5 .mu.g/cubic meter to 0.5 mg/cubic meter. Current federal and state guidelines for cleaning up contaminated dusts and soils range between 100 and 750 ppm (Reagan & Silbergeld, Trace Substances in Environmental Health in Environmental Geochemistry & Health (1990) 12, Suppl., 199-238). The FDA is proposing a lowered level for allowance leaching of lead from ceramic vessels.
At present there are no acceptable methods for identifying lead in humans or sources without laboratory based analytic support, almost always AAS, a resource intensive process. More significantly, for purposes of early detection and prevention, the need to send samples to a laboratory introduces substantial delays between sample collection and results. In screening programs the delay can be 1-3 months making it difficult to develop timely intervention and to implement treatment programs. Universal screening of all children is likely to be recommended. The impact on state and local screening programs will be enormous. Less than 5% of all children under 6 years currently are screened and almost all of the screening is done with EP as the first tier. The CDC have no recommendations as to alternate methods aside from venous blood and AAS and thus development of a new screening method is one of the highest priorities for eliminating lead poisoning (Public Health Service, 1991, Strategic Plan for Elimination of Childhood Lead Poisoning).
The criteria for a screening and detection method for lead exposure and of lead sources are those that relate to all such methods. First, the methods must be reliable and specific, that is, measure lead and not other metals, ions, or substances. The assays should not be confounded by commonly occurring conditions. Second, the assays must be appropriately sensitive, that is, generate a signal that detects lead or lead exposures at levels of current concern (for instance, in blood, concentrations of lead as low as 2-5 .mu.g/dl; concentrations in water as low as 1 ppb ) .
Additional criteria for screening and detection methods to be useful in public health include: the methods should produce results within the constraints of clinic and outpatient screening, that is, should utilize small amounts of sample (for instance, a fingerstick sample of blood or a single urine collection); should provide answers quickly within the clinic setting; should be relatively inexpensive; should be stable within a range of environmental settings, such as temperature; should be usable by persons with minimal technical training; and should be portable.